With the increasing cost of conventional energy sources, such as coal, oil, and natural gas, attention has been directed to harnessing solar energy. Substantial gains have been made in the development of photovoltaic technology, but the cost per unit of usable energy continues to be excessive. Contributing factors of such high cost are found in the cost of production of photovoltaic devices, primarily, cost of material and man-hours for assembly, maintenance costs and reliability of extended performance.
Advances in the development of the photovoltaic cell itself have been forthcoming. Indeed, the production of amorphous solar cells, such as amorphous silicon cells, has reduced considerably the material cost of photovoltaic assemblies. For a more detailed discussion regarding the development of amorphous solar cells, reference may be made to U.S. Pat. No. 4,409,605, to Ovshinsky et al.
An inherent problem with any type of solar cell involves the collection of electrical energy from the solar cell itself. Particularly, the electrical connection to the surface exposed to solar radiation is of major concern. This is so because any electrical collector means must ideally exhibit good electrical conductivity with high solar radiation transparency. Unfortunately, these two parameters are not found in any one material which is economical to solar cell production. As such it has been necessary to employ at least two distinct electrical conductor means to collect the electrical energy from the solar cell.
One arrangement normally employed in solar cell design involves a transparent conductive oxide (TCO) layer, such as indium tin oxide or indium oxide, layered over the top layer--either P or N type--of the semiconductor region of the solar cell. The TCO layer permits the influx of solar radiation into the semiconductor region while providing an electrical path therefrom. However, the electrical resistivity of the TCO layer is not suitable for current collection over large surface areas, i.e., greater than approximately 2 cm.sup.2. To remedy this problem, it is necessary to provide front contact electrical conductor grids of lower electrical resistance adjacent to, and electrically communicating with, the TCO layer to receive the electrical current therefrom.
Several techniques are presently employed for fabricating the front contact grid of photovoltaic cells. These techniques involve the fabrication of the grid directly onto the transparent conductive layer of the cell. Exemplary of this in situ fabrication of the grids are screen printing with conductive ink, and electroform plating using a mask to form a preselected pattern. These techniques, in addition to being time consuming from a production aspect, present inherent problems in the fabrication of photovoltaic cells. To begin with, the material costs for conductive inks--such as silver ink--are high. Furthermore, the grids thus formed are not readily electrically interconnected when multiple cells are assembled into a photovoltaic array. The small area of the grids, coupled with the material used, for example silver ink for screen printing, precludes favorable soldering or welding of the interconnectors to the grid.
Efforts have been made to form front contact current collector grids by methods other than in situ fabrication. Exemplary of these efforts is U.S. Pat. No. 4,348,546 to Little wherein a mesh of fine wires are encapsulated between a transparent cover plate and the front surface of the semiconductor strata during fabrication of the solar cell. Use of the fine wire as a front contact current collector for solar cells is also discussed in "An Alternative to the `Five Year Research Plan . . . 1984-1988 . . . Photovoltaics: Electricity from Sunlight` . . . U.S. Department of Energy, May 1983", The Grindelwald Letter, March 1984 edition (Eleventh in a series), published by Alfred H. Canada, Mammoth Lakes, Calif.
The use of the interconnectors, themselves, is not without inherent problems. Particularly, the interconnectors employed are highly susceptible to mechanical and thermal stresses experienced by the solar module. Indeed, the normal handling of the solar module during fabrication, installation and maintenance may easily damage the electrical connection network. In addition, the cyclic temperature change which the solar module experiences, as it is exposed to periods of sunlight and periods of darkness, causes thermally induced stresses in the electrical interconnectors which may ultimately result in fatigue failures thereof.
Efforts to remedy these stress problems have resulted in the general acceptance of electrical interconnectors of substantial size which are more resistant to the stress loadings. However, the larger size of the interconnectors causes increased shadowing of the solar cells in those areas where they overlay, thereby detrimentally reducing the output of electrical energy from the solar cell. To avoid such optical obscuration losses caused by the shadowing effect on the solar cell, the electrical interconnectors in some solar cells have been joined to the peripheral edge of the solar cell. Exemplary of this type of cell interconnection is U.S. Pat. No. 4,336,648, to Pschunder et al. However, this too causes detrimental reduction in the electrical output of the solar cell as current received, by the TCO layer and/or the front contact grid at the distal end from the interconnection junction is required to travel the full width of the solar cell through poorly conductive materials.
In addition to the foregoing problems, the process of accurately locating and securing the electrical interconnectors relative to each individual solar cell in the array is a time consuming process. These production difficulties have been a significant obstacle in the development and commercialization of photovoltaic devices.
Despite the substantial work and research conducted in the production of photovoltaic cells and array modules, no practical solutions have been developed to alleviate the aforesaid problems in fabricating the cells and array modules.